In the field of rechargeable batteries or electrochemical cells where metal ions are shuttled between cathode and anode at varying voltages, the initial source of metal ions (usually alkali metal) is typically the cathode material. An example of said metal ions includes lithium.
During the initial cycling of a lithium ion rechargeable battery, passivation films are formed on the anode and cathode, but particularly on the negative electrode. As shown in FIG. 1, several reactions can take place as this film is formed on the negative electrode, including solvent reduction, salt reduction, insoluble product formation, and polymerization. The passivation film is often referred to as an SEI layer (solid electrolyte interphase), the formation of which results in the loss of metal ions through an irreversible reaction, as well as a significant loss in battery capacity. Most often, lithium ion batteries are described as having an irreversible initial loss of 10 to 30%. A second type of irreversible loss of metal ions (e.g. lithium+) is due to side reactions that occur during the “shuttling” of metal ions during each additional charge and discharge cycle of the metal ion battery. A third type of irreversible loss is represented by a cathode passivation layer formation composed of soluble and insoluble lithium salts.
Precautions are taken to limit all types of irreversible losses (SEI, cathode passivation layer, and side reactions during long cycling). It would be advantageous, however, if a source could be provided to compensate for the excess metal ion requirement, in an amount necessary to support long cycle life. In most commercial metal ion battery systems, this reserve is provided by the cathode, and therefore the cathode must necessarily be sized to be about 135 to 150% of the specified discharge capacity of the battery, thus increasing the total weight of the battery. Once the irreversible loss of metal ions related to SEI and cathode passivation layer formation is complete, up to 30% of the cathode's metal-donating material has become “dead weight”, or non-operating material. Examples of these heavy and expensive cathode materials are LiFePO4, LiMn2O4 etc.
There have been attempts to source lithium metal to the anode during the construction of the anode. For example, FMC Corporation (Philadelphia, Pa.) has developed a stabilized lithium source called stabilized lithium metal powder, or SLMP (U.S. Pat. No. 8,021,496). This material can be mixed into carbon before an activation step, such as crushing or dissolving by the electrolyte (U.S. Pat. No. 7,276,314). However, SLMP is a very expensive lithium source compared to even common cathode donating materials, and may be difficult to distribute evenly.
Another example of sourcing metal to the anode is found in Li/polymer batteries, where Li metal is placed on a current collector to form an anode containing all the required overcapacity. The coulombic efficiency of this approach, however, is low when compared to the graphite anode based gel or liquid electrolyte battery approach. Furthermore, while the specific capacity is the highest possible, the cost of lithium metal foil is fairly high and the discharge rates for the necessary solid polymer electrolytes are low.
Others have attempted to increase the amount of alkali metal that is available during charge/discharge of an electrochemical cell using a process called pre-lithiation, first charging, or pre-charging, wherein a passivation film is either chemically or electrochemically formed on the anode prior to final assembly of the battery (U.S. Pat. No. 5,595,837; U.S. Pat. No. 5,753,388; U.S. Pat. No. 5,759,715; U.S. Pat. No. 5,436,093; and U.S. Pat. No. 5,721,067). In the cases where electrochemical pre-lithiation was conducted, either a lithium-containing electrode (most often consisting of elemental lithium metal), or a lithium foil was employed as the source of lithium. An alternate process that circumvents the formation of a passivation film, and thus the need to use pre-lithiation, is disclosed in U.S. Pat. No. 5,069,683.